Digital data transmissions over wired and wireless links may be corrupted, for instance, by noise in the link or channel, by interference from other transmissions (e.g., radio transmissions), or by environmental factors related to, for example, the speed, direction, location and requests between transmitting and receiving units. Even with clear communication channels (i.e., channels with limited corruption), which lend themselves to relatively high data rates, it may not be possible to appropriately decode a data stream with the requisite error rates. Digital data transmissions may also be limited by an inability of the receiving or transmitting equipment to appropriately encode and decode the data stream at the desired speed with the requisite error rate.
In other situations, it may not be feasible to provide hardware suitable for high rate data transmission at a cost and portability demanded by the application. Requested services may range from voice communications over high-speed Internet connections to video conferencing. The hardware at the receiver should be light and use minimal amounts of power in portable applications. Similarly, digital signal processing hardware for accurate conveyance of data packets should be compact and consume low power. Portability restrictions may require that all system attributes be well designed, using a minimal amount of integrated circuits, electronic components, batteries, and other components.
Error detection and correction codes typically provide mechanisms necessary to reliably receive and decode data packets. Forward error correction (FEC) codes allow decoders to accurately reconstruct data packets received with possible errors at the expense of some additional overhead (e.g., extra parity bits, extra symbols). Forward error protection may protect a data packet. With FEC, the protected data packet is generally “self-decoding” in that all the data (information) required to reconstruct the data packet is within a single receive block. Stored data may rely on FEC for reliable extraction. In two-way systems, an opportunity for requesting that a data packet be retransmitted upon detection of an error may be available. For example, an automatic repeat or retransmission request (ARQ) may be sent upon detection of an error using, for example, a parity bit check or a cyclic redundancy check (CRC), and then the original data packet may be discarded. Upon receipt of an ARQ request at the sending station, the packet may be retransmitted in its original form.
While this simple combination of ARQ and FEC is sometimes called Type I ARQ, the term “hybrid ARQ” is usually reserved for a more complex procedure where a receiver may combine previously received erroneous packets with a newly received packet in an effort to successfully ascertain the contents of the packet. The general procedure in a hybrid automatic repeat request (HARQ) system is that a receiver may generate an indicator, such as an ARQ request, upon detection of an error in the received data packet. Unlike Type I ARQ, the receiver does not discard previously received erroneous packets. The receiver may keep (e.g., store) the entire or portions of the erroneous packet because the erroneous packet may still contain worthwhile information, and therefore the erroneous packet is not discarded when using HARQ. By combining erroneous packets, the receiver may be able to assist the FEC to correct the errors. The receiver may require, however, an abundance of volatile memory for storing previous data packets and provisioning for the needs of the HARQ decoder. A HARQ buffer may contain a large amount of memory designated for just this purpose. Large memory requirements, particularly in portable, mobile equipment, may require excessively large space and consume an excessively large amount of power. Managing and optimizing volatile memory available in a communication system is thus desirable.
As demands for higher data rates increase, techniques for inserting more data into a single or multiple channels become more attractive. Adaptive modulation and control (AMC) techniques adjust the modulation scheme used for the transmission of data packets. A clear channel, for example, may use a higher order of modulation, e.g., 64-Quadrature Amplitude Modulation (64-QAM), to transmit data at the higher rate. A noisy or possibly faded and/or interference-limited channel may require the use of a lower order of modulation and consequently a lower data rate. When the channel improves, a high order modulation technique may be used again. Multiple input, multiple output (MIMO) transmission techniques utilizing multiple antennas for transmitting and receiving radio-frequency signals enhance the data rates that are possible using multiple channels. Effective HARQ architectures are needed to extract the full advantages of AMC and MIMO for higher data rates, while retaining compatibility with ARQ and HARQ systems.
In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communications protocol development, a compliant user terminal is typically allocated spectrum resources on a single compliant carrier in the downlink direction (i.e., from a base station to a user terminal). The uplink spectrum resources for uplink transmissions (i.e., from the user terminal to the base station) may be on a different single compliant carrier for Frequency Division Duplex (FDD) systems or on the same single compliant carrier for Time Division Duplex (TDD) system. For such a compliant user terminal, there is an expected minimum number of soft buffer locations, or a minimum HARQ buffer size that the UE is expected to provide. The cellular base station may then, based on the HARQ buffer size, be able to determine which codeword bits to the send to the user. Typically, the base station may determine the amount of storage per codeword based on the total number soft buffer locations, the number of HARQ processes and the number of layers for spatial multiplexing.
It is anticipated that some wireless communications protocols will support spectrum aggregation wherein a compliant user terminal, also referred to as user equipment (UE), will be expected to receive data on multiple component carriers in a single sub-frame. One such protocol is the 3GPP LTE-Advanced (LTE-A) protocol. Existing control signaling schemes for LTE Release 8 (Rel-8) can be used to allocate resources to a UE on only a single Release 8 compliant carrier. In LTE Rel-8/9, the UE decides the soft buffer size for each TB (Nir) using the following formula provided in 3GPP TS 36.212 Rel-8/9 specification:
                              N          IR                =                  ⌊                                    N                              soft                ⁢                                                                                                                      K                MIMO                            ·                              min                ⁡                                  (                                                            M                                              DL                        ⁢                                                                                                  ⁢                        _                        ⁢                                                                                                  ⁢                        HARQ                                                              ,                                          M                      limit                                                        )                                                              ⌋                                    Eqn        .                                  ⁢                  (          1          )                    
where Nsoft is the total number of soft channel bits (from Table 4.4-1 in TS 36.306), KMIMO is equal to 2 if the UE is configured to receive PDSCH transmissions based on spatial multiplexing with rank greater than 1 such as transmission modes 3, 4 or 8, 1 otherwise, MDL—HARQ is the maximum number of DL HARQ processes (i.e., HARQ processes in the downlink direction), and Mlimit is a constant equal to 8. For TDD, when the number of DL HARQ processes exceeds 8, techniques such as soft buffer overbooking, equal soft buffer split between HARQ processes, and/or other statistical buffer management techniques are applied. For Rel-10 with Carrier Aggregation (CA), new UE categories with support for 2 Component Carriers (CCs) need to be defined. Other UE categories with support for larger number of CCs may be defined for later releases. The bandwidth (BW) of individual aggregated CCs can be same or different (e.g., 10 MHz+10 MHz, 15 MHz+5 MHz etc.).
For LTE Release 10 with CA, new UE categories with support for 2 component carriers are yet to be defined. Also, other UE categories with support for larger numbers of component carriers may be defined for later releases. The bandwidth of the individual aggregated component carriers can be same or different (e.g. 10 MHz+10 MHz, 15 MHz+5 MHz etc.). Thus, there is a need for a soft buffer management wherein the UE may receive downlink transmissions from the eNB (or a plurality of eNBs) on one or more of the multiple component carriers.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.